U.S. patent number 7,780,765 [Application Number 12/164,400] was granted by the patent office on 2010-08-24 for control of mercury emissions from solid fuel combustion.
This patent grant is currently assigned to ALSTOM Technologies Ltd. Invention is credited to Kurt W. Johnson, Shin G. Kang, Srivats Srinivasachar.
United States Patent |
7,780,765 |
Srinivasachar , et
al. |
August 24, 2010 |
Control of mercury emissions from solid fuel combustion
Abstract
A method for removing mercury from flue gases generated by the
combustion of coal comprises: storing a starter batch of activated
carbon in an agglomerated state; de-agglomerating the starter batch
in a separation device to create a contact batch of activated
carbon; transporting the contact batch to a contact location;
injecting the contact batch into contact with the flue gas at a
contact location having a temperature between 400.degree. F. and
1100.degree. F., whereupon the activated carbon of the contact
batch adsorbs mercury from the flue gas; and removing the activated
carbon having mercury adsorbed thereon from the flue gas. The
transporting step is conducted with substantially no intermediate
storage of the contact batch following the de-agglomeration of the
starter batch to prevent re-agglomeration of the activated carbon
prior to injection.
Inventors: |
Srinivasachar; Srivats
(Sturbridge, MA), Kang; Shin G. (Simsbury, CT), Johnson;
Kurt W. (East Windsor, CT) |
Assignee: |
ALSTOM Technologies Ltd
(CH)
|
Family
ID: |
35447937 |
Appl.
No.: |
12/164,400 |
Filed: |
June 30, 2008 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20090056538 A1 |
Mar 5, 2009 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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10961697 |
Oct 8, 2004 |
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10453140 |
Jun 3, 2003 |
6848374 |
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Current U.S.
Class: |
95/134; 110/203;
423/210; 110/345; 95/58 |
Current CPC
Class: |
B01J
20/20 (20130101); F23J 15/006 (20130101); F23J
15/003 (20130101); B01D 53/64 (20130101); B01D
53/10 (20130101); B01D 53/83 (20130101); B01J
20/28004 (20130101); F23J 2215/60 (20130101); B01D
2257/602 (20130101); F23J 2219/60 (20130101); F23J
2217/102 (20130101); B01D 2253/102 (20130101) |
Current International
Class: |
B01D
53/02 (20060101) |
Field of
Search: |
;95/58,107,134
;110/203,345 ;423/210 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0253563 |
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Jan 1988 |
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EP |
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1275430 |
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Jan 2003 |
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EP |
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93/20926 |
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Oct 1993 |
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WO |
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2004/108254 |
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Dec 2004 |
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WO |
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Other References
Carey, Todd R. "Assessing Sorbent Injection Mercury Control
Effectiveness in Flue Gas Streams", Environmental Progress,
American Institute of Chemical Engineers, US vol. 19, No. 3, 2000
pp. 167-174. cited by other.
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Primary Examiner: Hill, Jr.; Robert J
Assistant Examiner: Jones; Christopher P
Attorney, Agent or Firm: Michaud-Kinney Group LLP
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation of U.S. patent application Ser.
No. 10/961,697, filed Oct. 8, 2004, which is a continuation-in-part
of U.S. patent application Ser. No. 10/453,140, now U.S. Pat. No.
6,848,374, filed Jun. 3, 2003, both of which are incorporated by
reference herein in their entirety.
Claims
What is claimed is:
1. A method for removing mercury from flue gases generated by the
combustion of coal, the method comprising: storing a starter batch
of activated carbon in an agglomerated state; de-agglomerating the
starter batch in a separation device to create a contact batch of
activated carbon; injecting the contact batch into contact with the
flue gas at a contact location between an economizer and an air
preheater, whereupon the activated carbon of the contact batch
adsorbs mercury from the flue gas; transporting the contact batch
to the contact location with substantially no intermediate storage
of the contact batch following the de-agglomeration of the starter
batch to prevent re-agglomeration of the activated carbon prior to
injection; and removing the activated carbon having mercury
adsorbed thereon from the flue gas.
2. The method of claim 1, wherein the transporting occurs in a
manner in which, on average, at least ninety percent (90%) of the
contact batch is delivered to the contact location in less than
five (5) minutes following milling.
3. The method of claim 1, wherein the separation device reduces the
size of the activated carbon in the starter batch such that the
activated carbon in the contact batch has a median particle size
(d.sub.50) less than 15 microns, where d.sub.50 represents 50% of
the particles by mass in the entire distribution in the contact
batch.
4. The method of claim 3, wherein the d.sub.50 of the contact batch
is less than 8 microns.
5. The method of claim 3, wherein the d.sub.50 of the contact batch
is no more than one-half (1/2) of a d.sub.50 of the starter
batch.
6. The method of claim 3, wherein the separation device includes at
least one of: a jet mill, a ball mill, and a roller mill.
7. The method of claim 1, further comprising: depositing at least
one of a halogen species and an acidic species on the contact batch
prior to disposing the contact batch into contact with the flue
gas.
8. The method of claim 1, wherein the activated carbon adsorbs at
least one of elemental mercury and mercury compounds from the flue
gas.
9. The method of claim 1, wherein the temperature of the flue gas
at the contact location is between 600.degree. F. and 800.degree.
F.
10. The method of claim 1, wherein the injunction location is
upstream of a flue gas side of the air heater and a collection
location is at one of a baghouse and an electrostatic
precipitator.
11. A method for removing mercury from flue gases generated by the
combustion of coal, the method comprising: milling a starter batch
of activated carbon to provide a contact batch of activated carbon
having a median particle size (d.sub.50) less than 15 microns,
where d.sub.50 represents 50% of the particles by mass in the
contact batch, the starter batch having a greater median particle
size than the contact batch; injecting the contact batch into
contact with the flue gas at a contact location between an
economizer and an air preheater, whereupon the activated carbon of
the contact batch adsorbs mercury from the flue gas; transporting
the contact batch to the contact location with substantially no
intermediate storage of the contact batch following the milling of
the starter batch to prevent agglomeration of the activated carbon
in the contact batch prior to injection; and removing the activated
carbon having mercury adsorbed thereon from the flue gas at a
removal location.
12. The method of claim 11, wherein the d.sub.50 of the contact
batch is less than 8 microns.
13. The method of claim 11, wherein the transporting occurs in a
manner in which, on average, at least ninety percent (90%) of the
contact batch is delivered to the contact location in less than
five (5) minutes following the milling.
14. The method of claim 11, further comprising: depositing at least
one of a halogen species and an acidic species on the contact batch
prior to disposing the contact batch into contact with the flue
gas.
15. A method for removing mercury from flue gases generated by the
combustion of coal, the method comprising: milling a starter batch
of activated carbon to provide a contact batch of activated carbon
having a median particle size (d.sub.50) less than 8 microns, where
d.sub.50 represents 50% of the particles by mass in the contact
batch, the starter batch having a greater median particle size than
the contact batch; injecting the contact batch into contact with
the flue gas at a contact location having a temperature between
600.degree. F. and 800.degree. F., whereupon the activated carbon
of the contact batch adsorbs mercury from the flue gas;
transporting the contact batch to the contact location with
substantially no intermediate storage of the contact batch
following the milling of the starter batch to prevent agglomeration
of the activated carbon in the contact batch prior to injection;
and removing the activated carbon having mercury adsorbed thereon
from the flue gas at a removal location.
16. The method of claim 15, wherein the transporting occurs in a
manner in which, on average, at least ninety percent (90%) of the
contact batch is delivered to the contact location in less than
five (5) minutes following the milling.
17. The method of claim 16, further comprising: depositing at least
one of a halogen species and an acidic species on the contact batch
prior to disposing the contact batch into contact with the flue
gas.
Description
BACKGROUND OF THE INVENTION
The present invention relates to apparatus and a method for
removing mercury from the products of solid fuel combustion
including flue gases and more particularly to apparatus and a
method for removing elemental mercury or mercury compounds from the
flue gases from coal combustion.
The use of activated carbon for the adsorption of mercury vapor has
been successfully demonstrated in various applications such as
municipal waste incineration. However, there are significant
differences in the concentration of mercury from waste incinerators
compared to coal-fired power plants with the concentration from the
coal-fired power plants being anywhere from 10 to 100 times lower.
Also, the mercury from waste incinerators is usually in the form of
mercury chloride whereas the mercury from coal-fired power plants
is usually in the form of elemental mercury. Both of these
differences make it more difficult to remove the mercury from the
flue gas from a coal-fired power plant.
The utilization factor for activated carbon is limited by the
relatively large particle size and low surface area which limits
the adsorption of mercury. Using activated carbon with mean
particle size of about 5 microns with a top size of about 10
microns would improve the mercury capture efficiency, but storage,
handling, transport and dispersion of these articles is extremely
difficult. As a result, the use of activated carbon for mercury
capture in coal-fired power plants is too costly. In such
applications, the utilization of the activated carbon is quite low
with a minimum mole ratio of carbon to mercury of 10,000 to 1.
Another form of carbon which has been proposed for the capture of
mercury from flue gases is carbon black. Carbon black is a finely
divided form of carbon produced by the incomplete combustion or
thermal decomposition of a hydrocarbon fuel. The most common form
of carbon black is referred to as furnace black or soot which is
made by burning natural gas or petroleum oil in a closed furnace
with about 50% of the air required for complete combustion. The
external surface area of the carbon black is about 100 times that
of activated carbon. This could result in a significant decrease of
the C/Hg mole ratio for effective mercury capture compared to
activated carbon. As the market price for carbon black is similar
to that for activated carbon, there is the potential for a
significant cost reduction.
Carbon black generation for the capture of mercury from a refuse
incinerator is disclosed in the International Patent Application
PCT/SE93/00163 (International Publication Number WO 93/20926). This
is characterized by the burning of a fuel with a deficiency of
oxygen in a separate incinerator and injecting the soot-laden flue
gas into the flue gas from the refuse incinerator. However, oxygen
deficient combustion leads to the generation of other pollutants
such as carbon monoxide and unburned hydrocarbons. Even if the flue
gas from the carbon black generator were to be injected upstream of
an acid gas removal device such as a flue gas scrubber, the carbon
monoxide and unburned hydrocarbons would not be destroyed or
removed.
Another problem with the application of prior art carbon black and
activated carbon-based methods for mercury capture from
incinerators to the capture of mercury from coal-fired power plants
is that refuse incinerators have high chlorine levels and the
mercury is present in the flue gas predominantly as mercury
chloride as previously stated. In coal-fired power plants, the
mercury is usually elemental. Although carbon black and activated
carbon have a relatively high affinity for the adsorption of
mercuric chloride, they have a relatively lower affinity for the
adsorption of elemental mercury.
Carbon-based sorbents such as activated carbon have been proposed
for controlling vapor phase mercury emissions in power plant flue
gases. In a conventional method, carbon sorbents are injected in
the flue gas duct upstream of particulate removal device such as
baghouses and electrostatic precipitators and downstream of air
heaters.
The temperature of the location at which carbon sorbents are
injected has heretofore been taken into consideration for the
reason that it has been suggested that the adsorption capacity of
mercury on carbon sorbents is higher at relatively lower
temperatures. In this connection, it is also known in the art that
physical adsorption (physisorption) of mercury on carbon is reduced
with temperature. Consequently, the prior art essentially does not
provide any detailed information concerning the desirability of
injecting, for the purpose of capturing mercury from gas streams,
carbonaceous sorbents at relatively higher temperatures such as,
for example, temperatures above about 400.degree. F.
It is also known in the art that vapor phase mercury in the flue
gas emerging from the high temperature boiler is in the form of
elemental mercury. Oxidation of elemental mercury to oxidized
mercury (Hg.sup.2+) is beneficial to mercury control since it can
be removed more easily by carbonaceous material.
The injection of activated carbon into the flue gas may typically
lead to a presence of, or an increase in, carbon in the fly ash,
whereupon an amount of carbon in the fly ash above a prescribed
limit may prevent the use of this fly ash for the purpose, for
example, of concrete manufacturing. The activated carbon, because
of its hydrophobic nature, adsorbs air-entraining additives that
are used in the concrete formulation. Also due to activated carbon,
the fly ash changes its color to dark grey to black. One way of
minimizing this air entrainment impact is to oxidize the carbon,
making it more hydrophilic. Another way to make the carbon
hydrophilic is by impregnating the carbonaceous sorbent with
additives that are hydrophilic, e.g., halide salts such as iron
chloride. Still another way of minimizing this problem is to reduce
the usage of carbon sorbent below a level that impacts air
entrainment behavior and discoloration.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method for the
capture of mercury from a coal-fired power plant with carbonaceous
sorbent without emitting pollutants generated in the production of
the carbonaceous sorbent. The invention also may involve the
treatment of the carbonaceous sorbent to enhance the capture of
elemental mercury.
In one aspect of the present invention, a method for removing
mercury from flue gases generated by the combustion of coal
comprises: storing a starter batch of activated carbon in an
agglomerated state; de-agglomerating the starter batch in a
separation device to create a contact batch of activated carbon;
injecting the contact batch into contact with the flue gas at a
contact location having a temperature between 400.degree. F. and
1100.degree. F., whereupon the activated carbon of the contact
batch adsorbs mercury from the flue gas; transporting the contact
batch to the contact location with substantially no intermediate
storage of the contact batch following the de-agglomeration of the
starter batch to prevent re-agglomeration of the activated carbon
prior to injection; and removing the activated carbon having
mercury adsorbed thereon from the flue gas.
In another aspect, a method for removing mercury from flue gases
generated by the combustion of coal comprises: milling a starter
batch of activated carbon to provide a contact batch of activated
carbon having a median particle size (d50) less than 15 microns,
where d50 represents 50% of the particles by mass in the contact
batch, the starter batch having a greater median particle size than
the contact batch; injecting the contact batch into contact with
the flue gas at a contact location having a temperature between
400.degree. F. and 1100.degree. F., whereupon the activated carbon
of the contact batch adsorbs mercury from the flue gas;
transporting the contact batch to the contact location with
substantially no intermediate storage of the contact batch
following the reduction of the starter batch to prevent
agglomeration of the activated carbon prior to injection; and
removing the activated carbon having mercury adsorbed thereon from
the flue gas at a removal location having a removal temperature
between 120 to 400.degree. F.
The present invention exploits the recognition that carbon sorbents
may also be used to effect chemisorption of mercury onto their
surfaces. Contrary to physisorption, the extent of chemisorption
tends to increase with temperature and the present invention thus
provides a process for exploiting the adsorption (both
chemisorption and physisorption) of mercury takes place at a wider
temperature range of 100.degree. F. to 800.degree. F.
Equilibrium calculations of potential mercury species present in
coal combustion flue gases show lower temperatures favor more
complete conversion to oxidized mercury. However, the present
invention has led to the recognition that kinetics of oxidation is
not favored as the flue gas is cooled, even though equilibrium
would indicate complete oxidation as well as the recognition that
oxidized mercury species (HgCl.sub.2) already begin to form at
temperatures lower than about 600.degree. C. (873 K) from elemental
mercury. In accordance with this recognition that higher
temperatures will favor kinetics of this transformation, the
process of the present invention utilizes the entire temperature
range of 300 to 1,000.degree. F. to oxidize and capture
mercury.
An object of the present invention is to reduce consumption of
carbon sorbent for both fabric filters and, in particular,
electrostatic precipitators for a prescribed degree of vapor phase
mercury removal.
Another object of the present invention is to increase the
oxidation degree of vapor phase mercury and decrease the amount of
vapor phase elemental mercury.
A further object of the present invention is to minimize the impact
of carbon sorbent injection on the utilization of fly ash.
Other objects and advantages of the invention will become apparent
from the drawings and specification.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention may be better understood and its numerous
objects and advantages will become apparent to those skilled in the
art by reference to the accompanying drawings in which:
FIG. 1 is a schematic diagram of a first embodiment of a system in
accordance with the present invention for removing elemental
mercury or mercury compounds from the flue gases from coal
combustion;
FIG. 2 is a schematic diagram of a second embodiment in accordance
with the present invention of a system for removing elemental
mercury or mercury compounds from the flue gases from coal
combustion;
FIG. 3 is a schematic diagram of a third embodiment of a system in
accordance with the present invention for removing elemental
mercury or mercury compounds from the flue gases from coal
combustion;
FIG. 4 is a schematic diagram of the ash disposal system of FIGS.
1-3;
FIG. 5 is a schematic diagram of an optional SO.sub.3 (sulfur
trioxide) sorbent addition subsystem;
FIG. 6 is a graph illustrating the effect of sorbent particle size
and halogen treatment on mercury capture;
FIG. 7 is a graph illustrating the effect of dispersion on in-situ
particle size distribution;
FIG. 8 is a schematic diagram of a fourth embodiment of a system in
accordance with the present invention for removing elemental
mercury or mercury compounds from the flue gases from coal
combustion;
FIG. 9 is a graph illustrating conversion (%) of soot, CO, and HC
as a function of temperature (.degree. C.);
FIG. 10 is a graph illustrating a typical destruction profile for
CO and HC with a PGM catalyst;
FIG. 11 is a graph illustrating examples of the exposure
temperature of sorbents in accordance with the present invention
for air heater-ESP, air heater-spray dryer-ESP and hot ESP
configurations;
FIG. 12 is a graph illustrating a thermo-gravimetric analysis of
the sorbent in the form of fine activated carbon in 5% O.sub.2 in
N.sub.2 to determine its ignition temperature;
FIG. 13 is a schematic illustration of an arrangement having a
sorbent injection location corresponding to the selectively
determined process limits;
FIG. 14 is a schematic illustration of an arrangement including an
SCR and having a sorbent injection location;
FIG. 15 is a graph illustrating the results of a trial to determine
mercury capture efficiency by a first group of injected carbon
sorbent that are deemed to be the group of in-flight sorbent
particles and a second group of injected carbon sorbent that are
deemed to be the group of deposited sorbent particles;
FIG. 16 is a schematic illustration of a plurality of multiple
injection ports provided to uniformly distribute sorbent particles
across the cross section of the flue gas flow;
FIG. 17 is a schematic illustration of a slurry injection for
enhanced sorbent deposition;
FIG. 18 is a schematic illustration of a precharging of sorbent
particles;
FIG. 19 is a perspective view of a small portion of the injected
sorbent as it coats the air heater elements and of the balance of
the sorbent particles as they are entrained by preheated air;
FIG. 20 is a schematic illustration of a combination of flue gas
side injection with air-side injection helps control mercury for
plant configurations with hot-side ESPs;
FIG. 21 is a schematic top plan view of the sequential operation of
a sorbent injection lance system for a rotary regenerative air
heater;
FIG. 22 is a schematic view of air-side injection of products from
an on-site sorbent generation and treatment unit; and
FIG. 23 is a schematic view of multiple sorbent injection for a
rotary regenerative air heater.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
With reference to FIG. 1, there is illustrated therein an exemplary
fuel conversion arrangement for converting a fuel--namely, a solid
fossil fuel in the form of coal--into a desired energy form. The
exemplary fuel conversion arrangement illustrated in FIG. 1 effects
the fuel conversion via combustion of coal and is illustratively
configured as a typical 100 MWe coal firing plant 10 which combusts
approximately 40 tons/hr of coal with a flue gas flow 12 of about
350,000 Nm.sup.3/hr. The raw coal is fed 14 to a pulverizer/crusher
16 where the raw coal is reduced to particulate size. Primary air
carries the coal particulates from the pulverizer/crusher 16 to the
boiler 18, where the coal is burned to convert water into steam.
The temperature of the flue gases leaving the boiler/furnace 18
ranges from 1400 to 2200.degree. F. The flue gases are cooled in
the superheater and convective pass 20 (economizer/re-heater) to
around 600 to 800.degree. F. before entering the air preheater 22.
Flue gas temperatures exiting the air preheater 22 and entering the
electrostatic precipitator (ESP)/fabric filter 24 range from 220 to
370.degree. F. If the sorbent requirement for mercury capture were
1 lb./MMacf, about 20 lb/hr of sorbent would need to be injected.
At 5 lb/MMacf (120 mg/Nm.sup.3), the sorbent requirement would be
100 lb/hr.
In a first embodiment of a system 26 for removing elemental mercury
or mercury compounds, a starter batch of carbonaceous sorbent is in
the form of carbonaceous sorbent 28 stored in a silo 30, with the
carbonaceous sorbent 28 being in an agglomerated state because the
very small particles thereof tend to stick to each other.
Accordingly, the starter batch of the sorbent 28 is fed by a feeder
32 to a separation device 34, which comminutes (if necessary) and
de-agglomerates the sorbent particles 28 into a contact batch of
carbonaceous sorbent and a retained batch of carbonaceous sorbent.
The contact batch of carbonaceous sorbent has a particle size
distribution of carbonaceous sorbent of d.sub.50<15 microns,
where d.sub.50 represents 50% of the particles by mass in the
entire distribution in the contact batch with the particle size
distribution of carbonaceous sorbent in the contact batch after
separation being less than the particle size distribution of
carbonaceous sorbent in the starter batch before separation and
less than the particle size distribution of carbonaceous sorbent in
the retained batch.
This device 34 may be a particle-particle separator or a jet mill,
where compressed air or high-pressure steam is the energy source.
The separation device 34 performs three functions:
particle-particle separation; particle size reduction; and
classification of fine particles to "product" and either return of
the coarse particles to the silo 30 or retention of the coarse
particles within the separation device 34 for further
processing.
The target particle size distribution is d.sub.50<15 microns,
preferably d.sub.50<8 microns and most preferably d.sub.50<4
micron, where d.sub.50 represents 50% of the particles by mass in
the entire distribution. Primary particle size reduction is
required when the starting particle size distribution is larger
than desired product size. No primary size reduction is required if
the primary particle sizes are already smaller than the target
product size, such as in the case of carbon black where the primary
particle size less than 1 micron. The minimum energy input for this
separation and optional size reduction device 34 is 10 kWh/ton,
more preferably it is 100 kWh/ton, and most preferably it is 1000
kWh/ton. This energy input can be supplied via high pressure fluid
such as steam or compressed air 36 in an air jet mill, or by media
such as grinding balls or rollers in a ball mill or roller
mill.
In addition to handling thereof by the separation device 34, the
sorbent particles 28 are subjected to one or more processes before
they are injected into the stream of flue gas. In one alternative,
the sorbent particles 28 are sent along a path 38 from the silo 30
to an oxidizer unit 40, where the particles are contacted by an
oxidizing agent 42 (e.g., ozone, hydrogen peroxide, hot air,
concentrated nitric acid), making the outer surfaces of the
particles hydrophilic. The treated sorbent may then be fed along a
path 44 to the separation device 34 or sent along a path 46 to a
blender 48 where a solution 50 is sprayed 52 on the sorbent
particles by a sprayer 54 to deposit a halogen on the surface of
the sorbent particles 28. The solution 50 is chosen from potassium
iodide, iodine dissolved in potassium iodide, alkali halides (e.g.,
NaCl), and halide salts (e.g., CaCl.sub.2), or halogen acids (e.g.,
HCl, Hl, HBr, HF) dissolved in water. A typical additive amount
results in a halogen concentration in the sorbent of about 0 to 5
percent. The treated sorbent is then fed along a path 56 to the
separation device 34.
A halogen 58, such as chlorine, bromine, iodine or fluorine, may
also be deposited on the sorbent 28 by vaporizing the halogen 58 in
a vaporizer 60 and condensing/adsorbing it on the sorbent 28. The
vaporized halogen may be injected along a path 62 into the sorbent
28 between the particle oxidizer 40 (described above) and the
feeder 32, between the feeder 32 and the separation device 34, or
onto the "fines" and de-agglomerated particles leaving the
separation device 34.
The effect of halogen treatment on mercury removal is shown in FIG.
6 and Table 1. FIG. 6 shows that treating coal sorbent 28 with
iodine (0 to 2.5 weight percent) significantly improves sorbent
performance. About 10 percent mercury removal was obtained with no
iodine treatment for "fine" coal sorbent, while about 95 percent
mercury removal was obtained with 2.0 percent iodine addition to
"fine" coal sorbent. Table 1 shows the effect of halogen treatment
for carbon black sorbent and activated carbon sorbent. For carbon
black with a surface area of 100 m.sup.2/g and at a sorbent
concentration of 50 mg/Nm.sup.3 in the flue gas, addition of iodine
to sorbent 28 (1 percent I.sub.2 in carbon after treatment)
improved the mercury removal performance from 20 percent to 100
percent. For activated carbon at a sorbent concentration of 100
mg/Nm.sup.3 in the flue gas, addition of iodine to sorbent (1
percent I.sub.2 in carbon after treatment) improved the mercury
removal performance from 75 percent to 90 percent.
TABLE-US-00001 TABLE 1 Mercury Removal Efficiencies from an Air
Stream with Various Sorbents and Sorbent Concentrations
(~200.degree. F.) Sorbent Concentration (mg/Nm.sup.3) Sorbent 0 20
35 50 100 Carbon Black (SA = 100 m.sup.2/g, d.sub.50 < 1 .mu.m)
0 <5 -- 20 30 Carbon Black (SA = 500 m.sup.2/g, d.sub.50 < 1
.mu.m) 0 <5 -- 20 -- Carbon Black (SA = 500 m.sup.2/g with 1%
I.sub.2 0 40 60 100 100 Carbon Black (SA = 100 m.sup.2/g with 1%
I.sub.2 0 20 -- 100 100 Activated Carbon (d.sub.50 = 18 .mu.m) with
1% I.sub.2 0 15 -- 60 90 Activated Carbon (d.sub.50 = 18 .mu.m) 0
-- -- -- 75 *SA = Surface area; d.sub.50 = Weight mean particle
size
FIG. 7 shows in-situ size distributions for two samples injected
into a duct with different levels of energy used for
particle-particle separation (de-agglomeration). The first sample
is a "coarse" commercial powdered activated carbon, with a mean
particle size (d.sub.50) of 18 .mu.m. The second sample is a "fine"
sorbent with a mean particle size (d.sub.50) of 3 .mu.m. When
either sample was injected into the duct with a low energy level
being used for particle separation ("poor" dispersion), the actual
size of particles observed in the duct was significantly coarser
than when higher energy was used for particle separation ("high"
dispersion).
The static pressure of air leaving the separation device 34 is
around 5 to 10 inches water gauge. This static head may be
insufficient to transport and distribute the sorbent 28 via the
injection lances into the flue gas duct. As seen in FIG. 1, a
"dirty air" material-handling fan/blower 64 may be placed after the
separation device 34 to increase the static head for sorbent
transport and distribution. A static head of about 30 inches water
gauge is preferred.
The inventive apparatus also comprises a means for disposing the
contact batch of carbonaceous sorbent into contact with the
products of fuel conversion at a contact location such that the
carbonaceous sorbent of the contact batch adsorbs mercury.
Accordingly, the de-agglomerated sorbent 28 and the conveying
air/steam is injected at a contact location 66 into the flue gas
duct through such a means in the form of a distributor 68 having
multiple lances. The injector tip is designed to impart a
tangential momentum to the sorbent stream (swirl tip) and increase
the rate of spread and distribution of the sorbent 28 in the flue
gas stream 12. The sorbent 28 may be injected into the flue gas
stream 12 between the boiler 18 and the convective pass/superheater
20, between the convective pass/superheater 20 and the air
preheater 22, or between the air preheater 22 and the ESP/fabric
filter 24.
Thus, the system 26 for removing elemental mercury or mercury
compounds handles carbonaceous sorbent 28 of a starter batch stored
in a silo 30 in an agglomerated state. The sorbent 28 is fed by a
feeder 32 to a separation device 34, which comminutes (if
necessary) and de-agglomerates the sorbent particles 28 to their
primary size distribution. The de-agglomerated sorbent 28 of a
contact batch created from the starter batch is then conveyed by
the airsteam for injection at a contact location 66 in a flue gas
duct whereat carbonaceous sorbent of the contact batch adsorbs
mercury from the flue gas. Preferably, the transport means of the
inventive apparatus which transports carbonaceous sorbent of the
contact batch to the contact location operates to deliver, on
average, at least ninety percent (90%) of the carbonaceous sorbent
of the contact batch to the contact location 66 in less than thirty
(30) minutes following the separation of the carbonaceous sorbent
of the starter batch into the carbonaceous sorbent of the contact
batch and the carbonaceous sorbent of the retained batch, more
preferably operates to deliver, on average, at least ninety percent
(90%) of the carbonaceous sorbent of the contact batch to the
contact location 66 in less than five (5) minutes following the
separation, and, most preferably, operates to deliver, on average,
at least ninety percent (90%) of the carbonaceous sorbent of the
contact batch to the contact location 66 in less than one (1)
minute following the separation.
In a second embodiment of a system for removing elemental mercury
or mercury compounds in accordance with the present invention and
shown in FIG. 2, hereinafter designated as a system 69 for removing
elemental mercury or mercury compounds, a portion of the coal
pulverized in the pulverizer 16 is extracted at a location 70 from
the pulverizer exit as sorbent 28. Preferably between 10 to 1000
lb/hr of coal (about 0.01 to 1.0 percent of total coal feed to
boiler), more preferably between 50 and 500 lb/hr, and most
preferably between 100 and 200 lb/hr is extracted at the location
70. A blower 72 may be required to provide the necessary motive
force for moving the extracted sorbent solids 28.
The extracted sorbent solids 28 are subjected to one or more
processes. The sorbent solids 28 may be sprayed at a location 74
with a solution 50 to deposit a halogen on the surface of the
sorbent particles 28. The solution 50 is chosen from potassium
iodide, iodine dissolved in potassium iodide, alkali halides (e.g.
NaCl), and halide salts (e.g. CaCl.sub.2), or halogen acids (e.g.
HCl, Hl, HBr, HF) dissolved in water. A typical additive amount
results in a halogen concentration in the sorbent 28 of about 0 to
5 percent. The treated sorbent 28 is then fed along a path 76 to
the separation device 34.
A halogen 58, such as chlorine, bromine, iodine or fluorine, may
also be deposited on the sorbent by vaporizing the halogen in a
vaporizer 60 and condensing/adsorbing it on the sorbent. The
vaporized halogen may be injected at a location 78 into the sorbent
28 between the blower 72 and the separation device 34, or onto the
"fines" and de-agglomerated particles leaving the separation device
34.
A "dirty air" material-handling fan/blower 64 may be placed after
the separation device 34 to increase the static head for sorbent
transport and distribution. The de-agglomerated sorbent 28 and the
conveying air/steam is injected into the flue gas duct through a
distributor 68 similar to that of the first embodiment 26. The
sorbent 28 may be injected into the flue gas stream 12 between the
boiler 18 and the convective pass/superheater 20, between the
convective pass/superheater 20 and the air preheater 22, or between
the air preheater 22 and the ESP/fabric filter 24.
In a third embodiment of a system for removing elemental mercury or
mercury compounds in accordance with the present invention and
shown in FIG. 3, hereinafter designated as a system 80 for removing
elemental mercury or mercury compounds, a portion of the coal
pulverized in the pulverizer 16 is also extracted at a location 82
from the pulverizer exit as sorbent 28. Preferably between 10 to
1000 lb/hr of coal (about 0.01 to 1.0 percent of total coal feed to
boiler), more preferably between 50 and 500 lb/hr, and most
preferably between 100 and 200 lb/hr is extracted at the location
82. A blower 84 may be required to provide the necessary motive
force for moving the extracted sorbent solids 28.
The extracted coal is partially combusted in a char generator
reactor 86 by subjecting the coal to temperatures between 300 and
1500.degree. C. The air supply to the char generator 86 is limited
to achieve only partial combustion of the coal, preferably only the
volatile portion of the coal. Accordingly, air amounts are
preferably controlled between 0.3 and 1.0 times the amount required
for complete combustion of coal, and more preferably between 0.5
and 0.7 times the amount required for complete combustion of
coal.
The high temperatures of the solids and gases exiting along a path
88 the char generator 86 are reduced in a downstream cooling
section 90. Water quenching or an air-, water- or steam-cooled heat
exchanger may be used to cool the gases and the solids. The
temperature of the gases and solids exiting the cooling section 90
is preferably around 300.degree. C. or lower.
The cooled gases and solids (char) are then sent along a path 92 to
a particle separator 94, preferably a cyclone. The separated gases,
with some fine particles, are discharged at a location 96 to the
main boiler 18 to ensure complete combustion of the unburnt
hydrocarbons and other combustible species like carbon monoxide and
carbon. The particles 28 separated by the particle separator 94 are
discharged along a path 98 to a silo 100. Alternatively, the
sorbent particles 28 may be discharged along a path 102 to an
oxidizer unit 104, where the particles 28 are contacted by an
oxidizing agent 42 (e.g. ozone, hydrogen peroxide, hot air,
concentrated nitric acid), making the outer surfaces of the
particles hydrophilic. The treated sorbent is then discharged along
a path 98 to the silo 100.
As the sorbent 28 is fed along a path 106 from the silo 100 to the
flue gas stream 12, a halogen is deposited on the outer surface of
the sorbent particles 28. In one alternative, the sorbent particles
28 are sent to a blender 48 where a solution 50 is sprayed on the
sorbent particles 28 to deposit the halogen on the surface of the
sorbent particles 28. The solution 50 is chosen from potassium
iodide, iodine dissolved in potassium iodide, alkali halides (e.g.,
NaCl), and halide salts (e.g., CaCl.sub.2), or halogen acids (e.g.,
HCl, Hl, HBr, HF) dissolved in water. A typical additive amount
results in a halogen concentration in the sorbent of about 0 to 5
percent. The treated sorbent 28 is then fed along a path 108 to the
separation device 34 by a feeder 110.
Alternatively, the halogen (e.g., chlorine, bromine, iodine or
fluorine) may be deposited on the sorbent by vaporizing the halogen
58 in a vaporizer 60 and condensing/adsorbing it on the sorbent 28.
The vaporized halogen may be injected at a location 112 into the
sorbent 28 between the silo 100 and the feeder 110, between the
feeder 110 and the separation device 34 or onto the "fines" and
de-agglomerated particles leaving the separation device 34.
A "dirty air" material-handling fan/blower 64 may be placed after
the separation device 34 to increase the static head for sorbent
transport and distribution. The de-agglomerated sorbent 28 and the
conveying air/steam is injected into the flue gas duct through a
distributor 68 similar to that of the first embodiment 26. The
sorbent 28 may be injected into the flue gas stream 12 between the
boiler 18 and the convective pass/superheater 20, between the
convective pass/superheater 20 and the air preheater 22, or between
the air preheater 22 and the ESP/fabric filter 24.
In a fourth embodiment of a system for removing elemental mercury
or mercury compounds in accordance with the present invention and
shown in FIG. 8, hereinafter designated as a system 114 for
removing elemental mercury or mercury compounds, the sorbent 28 has
the most preferable particle size mercury removal system--namely,
d.sub.50<2 micron, where d.sub.50 represents 50% of the
particles by mass in the entire distribution--and this is achieved
in this fourth embodiment by configuring the sorbent 28 as carbon
black or soot. Compared to "coarse" and "agglomerated" activated
carbon, much smaller quantities of carbon black/soot are required
to capture a prescribed amount of mercury in boiler flue gases. For
example, to remove 90% of elemental mercury, 50 mg/Nm.sup.3 of
impregnated carbon black are required compared to greater than 1000
mg/Nm.sup.3 of "coarse" and "agglomerated" activated carbon. Also,
much smaller quantities of oxidizing agents such as iodine would be
required for impregnation of the sorbent 28 (less than 1% by weight
of carbon) compared to activated carbon, where 1-10% by weight of
carbon would be required.
In a sub-assembly 116 of this system 114 for generating the sorbent
28 with a desirably very fine particle size distribution, a
soot-generating device 118 is disposed within the high temperature
region of the boiler 18. An oxidizing region 120 of the boiler 18,
downstream of the soot device 118, ensures that the CO and HC
generated during the soot generating process are destroyed. With
reference to FIG. 9, the temperature range in the oxidizing region
is preferably 500 to 1000.degree. C., and more preferably 600 to
800.degree. C., to ensure minimal soot destruction and maximal
CO/HC destruction.
In an alternative sub-assembly 122 of this system 114 for
generating the sorbent 28 with a desirably very fine particle size
distribution, the carbon black/soot is generated in a separate soot
generating device 124 having a CO/HC oxidation chamber 126. The
CO/HC oxidation chamber 126 may have an input 128 for receiving
oxygen or air required to destroy the CO/HC. The residence time of
the stream of carbon black/soot within the CO/HC oxidation chamber
126 and the temperature maintained in the CO/HC oxidation chamber
126 (preferably 500 to 1000.degree. C., and more preferably 600 to
800.degree. C.) are optimized to destroy the CO/HC with minimal
soot destruction.
In still another alternative sub-assembly 130 of this system 114
for generating the sorbent 28 with a desirably very fine particle
size distribution, a catalytic reactor 132 for targeted CO and HC
destruction is disposed between the soot generating device 134 and
the flue gas stream 12. One example of such a catalytic reactor 132
is a monolith (e.g. made of stainless steel or ceramic) coated with
a variety of platinum group metals (e.g., platinum, rhodium,
palladium). A typical destruction profile for CO and HC with a PGM
catalyst is shown in FIG. 10. At the temperatures shown in FIG. 10,
soot would not be destroyed to any significant extent.
In conventional coal-fired plants, unburnt carbon arriving at the
electrostatic precipitator (ESP) or fabric filter is predominantly
in the form of the large coal particles that have not finished
combustion. Fine particles of coal are rarely present in the normal
application since they are completely burnt out. In conventional
plants, therefore, the ash collected in the precipitator or fabric
filter consists of predominantly larger carbon particles and
smaller fly ash particles, which are very difficult to
separate.
In the mercury removal systems 26, 69, 80, 114 described above, the
carbonaceous mercury sorbent 28 is generally manufactured external
to the boiler/main combustor 18 and then subsequently reduced in
size and then introduced into the flue gas stream 12. The sorbent
particles 28 are engineered to be extremely small and are therefore
distinct from the ash and may be separated from the normal ash in
the ash disposal system 136 shown in FIGS. 1-3.
With reference to FIG. 4, the mercury-laden sorbent 28 is collected
in the ESP/fabric filter 24, along with fly ash 137. The
mercury-laden sorbent-fly ash mixture is transferred along a path
138 from the hopper 140 of the ESP/fabric filter 24 to an ash
storage silo 142 and then fed along a path 144 with a feeder 146 to
a classifier 148, which is capable of separating the "fine" and
low-density mercury sorbent 28 from the coarser and denser fly ash
137. The classifier 148 is preferably of the dynamic type with a
separately controlled classifier wheel, operating at tip speeds of
preferably greater than 50 m/s and more preferably 100 m/s to
ensure good separation and product recovery.
A particle-particle separator 150 may be disposed between the silo
142 and the classifier 148. The function of the particle-particle
separator 150 is to separate the mercury-laden sorbent 28 from the
fly ash 137, which would have agglomerated as a result of being
collected together in the ESP/fabric filter 24 and stored in the
hopper 140 and silo 142. A jet mill can be used for this purpose,
although, little size reduction is required and therefore the
energy consumption would be lower than if the particle size has to
be reduced.
As shown in FIG. 1, a SO3 (sulfur trioxide)-capturing sorbent 152
may be injected at a location 154 into the flue gas stream 12 along
with or separate from the mercury sorbent 28. With additional
reference to FIG. 5, a system 156 for injecting the SO3 (sulfur
trioxide)-capturing sorbent 152 includes a silo 158. The SO3
(sulfur trioxide)-capturing sorbent 152 stored in the silo 158 is
metered to the separation device 34 by a feeder 160. The
de-agglomerated particles 152 are then injected into the flue gas
duct by the blower 64 and distributor 68. A separation device,
blower and distributor which are separate from the ones used by the
mercury sorbent 28 may also be utilized (not shown). Alternatively
a SO3 (sulfur trioxide)-capturing sorbent precursor 162 may be
stored in the silo 158 and fed to a precursor converter 164. For
example, limestone 162 may be stored in the silo 158 and converted
to lime 152 in a calciner 164. It should be appreciated that the
SO3 (sulfur trioxide)-capturing sorbent system 156 may be used with
or separate from any of the mercury removal systems 26, 69, 80,
114.
In accordance with the present invention, treatment of the
carbonaceous sorbent with additives may be undertaken to further
improve the oxidation and capture of mercury. These additives
include halogens (chlorine, bromine, iodine, fluorine) and halides
(examples include ammonium chloride, ammonium bromide, iron
chloride, iron bromide, zinc chloride), H2SO4 and H3PO4. These
additives may be added (for example by spraying) to the
carbonaceous material as a water-based solution or another solvent
(such as alcohol)-based solution followed by evaporation of the
water/solvent. An example for incorporating the additive into the
carbonaceous material via a solution is iron chloride.
Additives may also be added by mixing them with the carbonaceous
sorbent and heating them to a temperature that will volatilize the
additive locally but distribute it by subsequent adsorption on the
carbon. It is preferred that the temperature to which the
carbonaceous material and additive are heated is above 400.degree.
F. and most preferably above 500.degree. F., to ensure that it will
be stable when injected in the flue gas at those temperatures. An
example of additive that can be incorporated in the above fashion
is iodine or bromine.
In accordance with the present invention, carbon-based sorbent is
injected at a location where interaction between injected sorbent
and mercury in flue gas is maximized both for (1) oxidation of
mercury on sorbent surface and for (2) its subsequent capture by
sorbent. The following three types of temperatures are preferably
taken into account in determining the sorbent injection location:
injection temperature, collection temperature and exposure
temperature range. In this regard, the injection temperature is
deemed to be the temperature of the location at which the sorbent
and the flue gas are first in contact with one another. Also, the
collection temperature is deemed to be the temperature of a given
collection location at which carbonaceous sorbent having mercury
absorbed thereon is separated from the flue gas either with or
without other solids, gases, or liquids entrained with the flue
gas. Accordingly, a given collection location may be a respective
known particulate removal device such as a cyclone, an
electrostatic precipitator (ESP), a baghouse, or a particulate
scrubber.
In connection with the capture of mercury in flue gas generated by
the combustion of fossil fuels such as coal, it is believed that
the sorbent injection temperature will typically be from 400 to
1100.degree. F., and the sorbent collection temperature from 100 to
800.degree. F. The exposure temperature range is bound by the
injection temperature--namely, the flue gas temperature at which
sorbent is injected--and the collection temperature--namely, the
flue gas temperature at which the majority of the sorbent is
removed from the flue gas. Additionally, the exposure temperature
range (injection temperature minus collection temperature) should
preferably be greater than 50.degree. F., preferably greater than
100.degree. F., and more preferably greater than 200.degree. F.
(temperature drop due to spray dryer excluded). FIG. 11 shows
examples of the exposure temperature of sorbents in accordance with
the present invention for air heater-ESP, air heater-spray
dryer-ESP and hot ESP configurations.
Trials have shown that, with respect to the injection of activated
carbon at temperatures higher than 400.degree. F. into a flue gas
obtained from the combustion of coal, mercury oxidation and removal
were higher than if was injected at lower temperatures.
There is an upper limit in temperature, however, to be taken into
account in selecting the injection point. The selection of this
limit takes into account the reaction of activated carbon with
oxygen in the flue gas at high temperatures, which results in the
consumption of the activated carbon. For example, subjecting
activated carbon to increasing temperatures in a gas stream
containing about 5 percent oxygen (typical for flue gas) may lead
to weight loss above approximately 750.degree. F. (see FIG. 12,
which is a graph illustrating a thermo-gravimetric analysis of the
sorbent in the form of fine activated carbon in 5% O2 in N2 to
determine its ignition temperature). This provides a qualitative
measure of the upper temperature limit. Since this exposure of the
carbon in the boiler flue gas is more appropriately measured in
seconds rather than minutes, it can be expected that the upper
temperature limit will be higher closer to 1000.degree. F.
These temperature limits identify the preferred temperature range
for carbon injection for mercury capture and oxidation. This range
is preferably between 400.degree. F. and 1100.degree. F. and more
preferably between 500.degree. F. and 900.degree. F. and most
preferably between 550.degree. F. and 750.degree. F. While these
temperature ranges are appropriate for the carbonaceous material
used in the description of this example, it is to be understood
that the above-identified temperature limits will differ for
different types of carbonaceous material, for the gas compositions
they are subjected, and the residence time the carbon is exposed at
the high temperature. Hence, efficacious capture of mercury in flue
gas via injection of a carbonaceous sorbent at relatively higher
temperatures should be expected to be constrained only by the
process limits such as noted above and not by the absolute specific
temperature targets.
An example of a sorbent injection location corresponding to the
above-noted process limits is schematically shown in FIG. 13. In
coal-fired boilers, coal is combusted to generate hot flue gas and
the hot flue gas passes through a series of heat exchangers and its
heat extracted to make superheated steam. The heat exchangers, in a
typical conventional configuration, may consist, in a first portion
thereof that is not illustrated herein, of a radiant furnace
section (for saturated steam production), followed by convective
superheater and reheater sections (for superheated steam
production). A further downstream portion of the heat exchanger
configuration, that is illustrated in FIG. 13, includes an
economizer section 310 for preheating the water (before it goes to
the radiant furnace for saturated steam production), and an air
heater section 312 (for preheating the combustion air). For the
limits identified, the optimum injection location of the
carbonaceous sorbent into the flue gas stream is a location 314
between the economizer and the air heater.
The carbonaceous sorbent injected in the flue gas is injected at
the exit of the economizer 310. It may be injected directly or
treated for size reduction and de-agglomeration in a mill 316 as
described below. In a variation of this embodiment of the process,
the carbonaceous sorbent is injected in two or more locations, one
of the locations being defined by the identified flue gas
temperature limits. For example, as shown in FIG. 13, in addition
to the injection location 314, a second injection location 318 can
be provided that is located downstream of the air heater 312 and
upstream of a particulate collection device 320.
Another example of a sorbent injection location is schematically
shown in FIG. 14, which illustrates a combustion and flue gas
handling arrangement comprising a selective catalytic reactor
(SCR). In this arrangement, the heat exchangers, in a typical
conventional configuration, may consist, in a first portion thereof
that is not illustrated herein, of a radiant furnace section (for
saturated steam production), followed by convective superheater and
reheater sections (for superheated steam production). A further
downstream portion of the heat exchanger configuration, that is
illustrated in FIG. 14, includes an economizer section 610 for
preheating the water (before it goes to the radiant furnace for
saturated steam production), and an air heater section 612 (for
preheating the combustion air). For the limits identified, the
optimum injection location of the carbonaceous sorbent into the
flue gas stream is a location 614 between the economizer 610 and a
selective catalytic reactor (SCR) 616.
The carbonaceous sorbent injected in the flue gas is injected at
the exit of the economizer 610. It may be injected directly or
treated for size reduction and de-agglomeration in a mill 618.
The injection of carbon sorbent in flue gas in accordance with the
present invention leads to two groups of carbon sorbent that can be
characterized by their respectively different residence times in
the boiler system: a first group of injected carbon sorbent that
are deemed to be the group of in-flight sorbent particles and a
second group of injected carbon sorbent that are deemed to be the
group of deposited sorbent particles. The in-flight sorbent
particles are defined as those suspended in the flue gas stream in
contact with mercury-containing flue gas and are characterized by
its residence time as short as 0.01 seconds to 30 seconds. The
deposited sorbent particles are those adhered onto the surfaces of
the boiler system and in direct contact with mercury in flue gas
and are characterized by its longer residence time, from 10 seconds
up to 10 days. (The upper limit may be determined by the
sootblowing cycle and shedding in ductwork.) From a simplistic
kinetics point of view, mercury capture by sorbent is expressed as:
Amount of mercury captured.varies.(total in-flight particle
surface)*(in-flight particle residence time)+(total deposited
particle surface)*(deposited particle residence time)
Of the two groups of carbon sorbent in the boiler system,
therefore, the more preferred group is the deposited sorbent
particles as they are in contact with mercury in flue gas much
longer than are the other group of particles. In accordance with
the present invention, the proportion of deposited sorbent
particles for the given amount of carbon sorbent injected, the
contact surface area of deposited sorbent particles and the
residence time of the deposited sorbent particles within the boiler
system are all respectively maximized or optimized.
FIG. 15 shows the contribution of these two groups to mercury
capture that were observed in a trial. The initial reduction of
mercury from 9 to 2 ug/Nm3 is due to in-flight sorbent and initial
sorbent deposit on the reactor wall. As more reactor surface is
covered by sorbent, mercury level further goes down to zero at a
slower rate as shown in the FIG. 15. As soon as the sorbent feeder
is turned off, the mercury level is recovered from zero to 4,
followed by slow recovery to 6. The recovery from zero to 4 is due
to lack of in-flight sorbents, and the recovery from 4 to 6 is due
to saturation of deposited sorbent with mercury.
In a typical coal-fired boiler system, the "backpass devices" that
provide a large surface area for sorbent deposition in the desired
temperature range of 100 to 1000.degree. F. are the air heaters,
the selective catalytic reactors (SCR), the electrostatic
precipitator (ESP) entrance nozzles, the ESP collection surfaces,
the flue gas ductwork and the fabric filters. In accordance with
the present invention, sorbent is injected upstream of any of these
devices to coat the surfaces provided by them with sorbent
particles. The following are examples of sorbent injection upstream
of these devices.
Air heaters are used for preheating primary and/or secondary air
and are an excellent device to provide a large sorbent deposit
surface area. In tubular air heaters, cold air flows over a bank of
tubes (shell side) through which hot flue gases pass. Mercury
sorbents can be injected into flue gases at a location upstream of
the air heater tube inlet so that sorbent particles are uniformly
distributed across the cross section of the flue gas stream before
they enter the heat exchanger tubes. A portion of sorbent particles
going through heat exchanger tubes is deposited (or coated) onto
the inner surface of tubes, either by Brownian motion or by
turbulent diffusion. Particles are held onto the surface mostly by
van der Waals force and interact with mercury in flue gases until
they are shed off by sootblowing mechanism or simply by flow around
them.
Sorbent injection lances are designed and installed upstream of the
inlet with spacing that ensures uniform coating of sorbent
particles on the heat exchanger tubes. For maximum utilization of
the heat exchanger tube surfaces, it is advantageous if, as shown
in FIG. 16, a plurality of multiple injection ports 410 are
provided to uniformly distribute sorbent particles across the cross
section of the flue gas flow. The injection ports 410 may be
configured as injection lances and each group of the injection
ports 410 are commonly communicated with a respective distribution
manifold 412. Uniform distribution of sorbent particles also
ensures good contact between in-flight particles and mercury in
flue gases.
Rotary regenerative air heaters are regenerative heat exchangers
with heat exchanger surface elements rotating between hot flue gas
duct and cold air ducts. Injection lance systems can be installed
upstream of the rotary regenerative air heaters in the flue gas
duct to coat the heat exchanger elements uniformly with sorbent
particles.
Sorbent deposition can be enhanced either by modifying the sorbent
surfaces or by modifying the surfaces of backpass devices, or both.
Sorbent surfaces can be treated with sticky material or its
precursors before injection. For example, surfaces of sorbent may
be treated with additives such as ammonium bromide, which become
sticky as it reacts with sulfur dioxide to form ammonium sulfate.
This sticky surface enhances sorbent deposition.
It can also be advantageous to prepare a slurry mixture of sorbent
(solid phase), a liquid phase such as water, and/or other additives
(either solid or liquid phase) that enhance mercury chemistry. The
slurry is then, as seen in FIG. 17, atomized/sprayed into the flue
gas, or directly onto surfaces of backpass devices. The liquid
phase of the slurry enhances deposition onto backpass surfaces. As
the liquid phase of the slurry evaporates, the residual phase
(sorbent and/or additives) on the surfaces of backpass devices
becomes exposed to flue gas for interaction with mercury.
Reference is had to FIG. 18 wherein another enhancement of the
process is illustrated wherein the sorbent particles are
electrically pre-charged before injection while, downstream of the
sorbent injection location, there is provided oppositely charged
(or ground) surfaces. The charged sorbent particles then
preferentially adhere to these surfaces and interact with mercury
for an extended period of time. This also allows selective
separation of pre-charged sorbent particles from the non-charged
fly ash stream, thereby avoiding any contamination of fly ash
stream with sorbent and allowing fly ash utilization for concrete
application.
Another way of avoiding (or minimizing) fly ash contamination while
achieving mercury reduction is by injecting sorbent to air side or
the neutral zone of rotary regenerative air heaters, rather than
flue gas side. In a typical rotary regenerative heater, most of the
fly ash particles (>90%) in flue gas pass through air heater. If
sorbent particles are injected into flue gas upstream of rotary
regenerative heater, most of the sorbent particles will pass
through the air heater (in-flight sorbent particles) and only a
small portion of the sorbent particles will deposit to the air
heater elements (deposited sorbent particles). Most of the injected
sorbent particles will participate in the mercury oxidation/capture
process for only a short period of time, i.e., 0.01 to 30 seconds,
and then end up in ash hoppers of an electrostatic precipitator
(ESP) or a baghouse contaminating fly ash stream.
The contamination of fly ash in ash hoppers can be advantageously
minimized by injecting sorbent to the upstream of the air side 502
of the air heater 504 (note that the sectors of this rotor of this
rotary regenerative-type air heater cyclically rotate through the
air side 502 into which fresh combustion air 506 and the flue gas
side 508 into which flue gas flows) or directly into the neutral
zone for coating the air heater elements with sorbent. As
illustrated in FIG. 19, a small portion 510 of the injected sorbent
512 coats the air heater elements 514, and most of the sorbent
particles are entrained by preheated air 516 and eventually end up
in the high-temperature burner zone and burn away.
The sorbent-coated air heater elements 514 in the air-side 502 then
rotate into the flue gas side 508 for mercury oxidation and
capture. The deposited sorbent particles eventually are dislodged
from the element surfaces by sootblowing, entrained by flue gas 518
and end up into the ash hoppers of an ESP or a baghouse. The amount
of sorbent into the ash hoppers in this case, however, will be
significantly less than that when sorbent is injected to the flue
gas side. In this fashion, contamination of fly ash in the ash
hoppers is significantly minimized.
The sorbent that passes through the air-side 502 of the air heater
504 is entrained into the boiler. As the carbonaceous sorbent burns
away, any additives in the sorbent are released into flue gas. If
the additives are halogen compounds such as chlorine, then the
added halogen will participate in the mercury chemistry enhancing
mercury oxidation and its capture.
Combinations of the air-side injection with the flue gas-side
injection can be used as well for further control of mercury as is
shown in FIG. 23.
The injection of sorbent to the air-side in accordance with the
present invention offers a viable mercury control solution to
plants with hot-side ESPs as well. A conventional way of mercury
control for this configuration is to inject sorbents upstream of
the ESP and capture them by the ESP. The technical difficulty in
this control method is that the sorbents get to experience the high
temperature zone for oxidation, but not the low temperature for
mercury capture. The sorbents may participate in the oxidation of
mercury but not its capture.
Sorbent injection to the air-side overcomes this difficulty. Air
heaters are located in a temperature range where the kinetics for
both oxidation and capture are fast. A combination of flue gas side
injection with air-side injection helps control mercury for plant
configurations with hot-side ESPs, as is shown in FIG. 20.
The rotation of the air heater elements at a constant speed makes
the injection lance system for air-side injection of sorbent
simpler than the one for flue gas side injection. The goal of
air-side injection is to cover as much surface of air heater
elements with sorbents as possible and as uniformly as possible.
One may design an injection lance system that covers the whole
cross section of the air sector. Since the air heater elements
rotate at a constant speed between the flue gas duct and the air
duct, however, one can achieve the goal with a simple injection
system with a few injection lances extending from the air heater
axis to the outer edge in a radial direction.
An example of the lance system is shown in FIG. 21, which is a
schematic diagram of the injection of sorbent at the rotary
regenerative air heater. Segments A, B, and C are the air-side of
the rotary regenerative air heater, whereas D, E and F are the flue
gas side. The air in A, B and C is flowing upwards whereas the flue
gas in D, E and F is flowing downwards. The heater elements are
rotating counterclockwise. In segment C, sorbent has been injected.
Some of it is deposited to the heater element surfaces (shown with
"x" cross-hatching) and most of it leaves the air heater along with
the combustion air. The sorbent-coated heater element enters the
flue gas side later and interacts with mercury in flue gas. In this
fashion, the sorbent transferred to the flue gas side is only that
deposited onto the surfaces.
Sorbent injection lance system for rotary regenerative air heaters.
At time T0, air heater segment A is in the air-side of the heater.
Injection lance BC is located in this case close to the inlet to
the flue gas side as the heater elements rotate counterclockwise.
Injection lance BC has multiple nozzles spraying sorbent into
general area of line segment BC uniformly. A time t1, the segment A
approaches the lance BC. At time t2, spraying sorbent to the
segment A has been completed. Finally at time t3, segment A is
rotating into the flue gas side. In this fashion, all of the air
heater elements are coated with sorbent before they enter the flue
gas side. The deposited sorbent on segment A interacts with mercury
in flue gas. The flue gas flow is into the page, and the airflow is
out of the pages.
For various reasons, one may opt to generate and/or treat sorbent
on site for mercury control. It may be advisable, in connection
with the preparation of a small amount of sorbent on site, to
ensure the proper environmental control of the undesirable
byproducts for the sorbent preparation process. Incorporation of
pollution control devices such as scrubbers or particulate control
devices dedicated to the sorbent preparation unit makes the on site
preparation less economical. A way of overcoming this on-site
generation issue is to operatively connect the outlet of on-site
generation unit to the upstream of the air-side rotary regenerative
heater, as is shown in FIG. 22.
In an on-site generation process, sorbents are generated as well as
undesirable byproducts. The byproducts in this case may be any
unused additives, intermediate hydrocarbon material produced in the
process, solvents or flue gases. As the output stream is directed
to the air-side, some sorbents will be deposited onto the air
heater element surfaces while the rest escapes the air heater. The
escaping stream, which includes most of the byproducts, will be
mixed with the combustion air ending up in the combustion zone of
the boiler.
In this way, byproducts from the on-site generation unit undergo
high-temperature destruction and are captured by the pollution
control devices of the boiler system.
Thus, it can be understood that the present invention offers the
advantages of lower sorbent consumption and costs when employed
with an electrostatic precipitator (ESP) and a baghouse, the
avoidance of high capital cost equipment (baghouse) downstream of
existing ESP, the removal of mercury for cases where no inherent
halogen is present in the flue gas (low chlorine fuels), the
generation of inactive (lower-activity) carbon in fly ash leading
to lower disposal costs and potential use in concrete, and the
provision of a cost effective solution to plant configurations with
hot-side ESPs.
While preferred embodiments have been shown and described, various
modifications and substitutions may be made thereto without
departing from the spirit and scope of the invention. Accordingly,
it is to be understood that the present invention has been
described by way of illustration and not limitation.
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